Sound detecting system



' acaasse on pg: srs'ram Thomas a. as. Scarsdaie, N. r, assignin to Westinghouse Electric @orporation, East Pittsburgh, Pa... a corporation oi? Zieyllyania Application some 24, rear, series at. 399,565

ion. arr-e52) This invention relates to sound detecting sys- I tems and, more particularly, to a system for detecting thepresence of aircraft "by means of the wave is automatically maintained irrespective of the magnitude of other vibrations which may be present in the vicinltyof the detecting apparatus. A particular advantage of a system of this type is that the sensitivity to minute sound intensities can be made extremely large, limited only by the critical value of amplification obtainable with vacuum tubes and associated components.

The detecting system herein described operates on the heterodype principle of wave reception and provides two distinct sound responsive elements.

. The individual response of any of these elements in the amplifier system fed therefrom produces no effect, whereas simultaneous response of the two elements to one source of sound produces proper frequency components by frequency conversion to be amplified and detected.

derlying principle of operation is that two separate sound pickup devices, such as microphones, are so positioned that the sound energy to be detected shall excite simultaneously both microphones. This can easily be achieved by suitable positioning and mounting of the microphones, of

which several may be connected to form two,

groups and so directed that one in each group converges with another one at a certain point.

In this manner, a large territory may be covered in which a sound source at any time will simultaneously energize one microphone in each group.

The microphones i and 2 are so' spaced as to be sumciently far apart so that sound from any source originating in the vicinity of one 'will not reach the other.

Referring to Fig. 1, two electroacoustical devices i and 2 are so positioned that their axes COD? verge at a far distance in the sky. These devices are preferably microphones of the type having directional characteristics which is indicated by the parabolic enclosures 3 and db The microphone l is shown to be connected to an amplifier which magnifies the sound energyoutput of the microphone and transmits this to a mixer stage. The other microphone 2 is similarly connected to an amplifier. However, the output thereof is fed to a balanced modulator. A local oscillator is provided which also feeds the italanced modulator. The output of the balanced modulator is applied to the mixer stage and the output of the mixer stage is fed to an intermediate frequency amplifier. The output of the latter is detected, that is rectified and the magnitude of the rectified component indicated by a suitable indicator.

The operation of the system will be described in more detail after a thorough understanding of the circuit andthe elements comprisingthe block seen from the description which follows, the undiagram components of the systemabove mentioned. Reference is to be had to Fig.2 in which the schematic diagram illustrates the circuit of the detecting system employing vacuum tubes in various functional applications. In order to simplify the illustration and make the operation easier to understand, individual batteries are shown for the power supply of each vacuum tube. It is to be understood, of course, that these bat teries may be replaced either by a common source of battery supply or by power supplies operating from alternating ordirect current. The filament circuit for heating the cathodes of .the tubes has also been omitted for the sake of simplicity in diagrammatic illustration, inasmuch as it is well known in the art that thermionic vacuum tubes require suitable sources of filament current.

The microphone i is shown here to be of the crystal variety although any other microphone may be used. It is connected between the grid 5 and the cathode 6 of the tub 7. The battery 8 provides the required bias for the grid electrode. The vacuum tube 7 represents one stage of an audio frequency amplifier oi the resistance-capacity coupled type.- The anode it connects to the load resistor it which is'in series between the cathode 8, the plate supply source 5 i and the filter I resistor l2. The Junction point of resistors l c and condenser H to the control grid l of the mixer tube IS. The input voltage for the grid l5 may be adjusted by means of the resistor of the volt-' age divider H, the return terminal of which is connected in series with the grid bias ource l3 to the cathode l3 of the tube I3. The output circuit of the mixer tube l3 between its anode 23 and cathode l9 includes the primary winding 2| of the' transformer 22 and the anode supply source battery 23. The primary winding 2| is tuned to resonance to a predetermined frequency by means of the condenser 24. The secondary winding 25 of the transformer 22 connects to the input circuit of the tube 26 comprising the grid 21 and cathode 23. The bias source 29 is in series with the secondary winding 25, the latter being also tuned to resonance to the same frequency by means of the condenser 33. The vacuum tub 23 represents the first stage of anintermediate frequency amplifier being tuned to a predetermined fixedfrequency. It'is followed by another stage similar in electrical details and construction. Consequently, the following stage needs no separate de-- scription and similar components thereof are in winding 53 of the transformer 51 in opposite phase to the grids 59 and 3|. Similarly, a fixed frequency energizing voltage is applied to the screen grids 31 and 68 of the tubes 33 and 32, respectively. This connection of the tubes 33 and 32 is known in the art as a balanced modulator circuit. whereby certain frequencies may be combined in a desired manner and others eliminated. Circuits of this type are described on page 121 of the book entitled Theory and Applications of Electron Tubes,by H. J. Reich, 1939 edition.

The other energizing voltage for the screen grids 3'! and 63 of the vacuum tubes 33 and 62, respectively, is obtained from a local' oscillator operating at a definite fixed frequency. The latter is shown as a crystal controlled vacuum tube oscillator comprising the vacuum tube 10, of

. which the grid 'Il is connected to the cathode 12 through a grid resistor I3. A piezo-electric crystal element 14 shunts the grid resistor and acts in a connection with ,the tuned output circuit as .the

' of a. center-tap, a push-pull input circuit. The v dicated by identical reference characters bearing primary indices. The vacuum tube 23 is of the screen grid type having its screen grid electrode 3| connected to an intermediate point on thepower supply represented by the batteries "and frequency determining element of the oscillator.

The oscillator anode 1515 connected to a tank circuit comprising the primary winding It of the transformer 11 tuned by the condenser 13, the anode voltage being applied between cathode 12 and anode I5 from the battery 19. The secondary winding 33 of the transformer l'l form by virtue free ends are connected to the screen'grids 31 and 33, respectively, and the center-tap to the voltage source represented by the battery 3| terminating at the cathode 34. I 1

In the arrangement herein shown, the mixer tube l3 and thebalanced'modulator tubes 63 and 32 are shown to be of the screen-grid type in which the screen grid is utilized alsoas'a second 33 in series, thelatter supplying also anode voltage between cathode 23 and anode 33 in series with the primary winding 35 of the transformer 22'. This winding is tuned by the condenser 33.

The output of the last stage of the intermediate frequencv amplifier is fed by means or the transformer 31 to a diode rectifier 33. The secondary winding 39 of the transformer 31 is connected to the anode 40 of the diode 33 and returns through the diode load resistor II in series with a current indicating instrument 42 to the cathode 43 of the diode 33. The secondary winding 39 is also tuned to resonance by the condenser 43. The intermediate frequency is by-passed by the condenser 45 connected between the junction point of the winding 39 and theresistor II and the cathode 43. V

The second microphone 2 is placed in the input circuit of another audio frequency amplifier comprising the tube 9 ofwhich the grid 53 connects to the rider 5| of the voltage divider 52 paralleling the terminals of the microphone 2. A bias source for the grid is indicated'by the battery 53 the positive terminal of which is concontrolgrid. This type ofconnection is merely for the purpose of simplification. Other types of tubes may equally be used, particularly those which-'have'auxiliary grids for the purpose of injecting'secondary control voltages besides the primarycontrol voltage applied to the signal grid. Tubes of this type are known as mixer 0r converter tubes. The RCA type 6L7 may be mentioned as an example.-

The output circuit of the balanced modulator tubes includes the anode 32 of the tube 30 and the load resistor 33, whereas, for the tube 62, the anode 34 and the load resistor 85. The junction point of the resistors 33 and 35 returns to the anode supply source represented by the battery 33 in series with a common load resistor, 31. Across the latter the voltage will be proportional to the sum of the plate currents of the modulator tubes. Since the output of the balanced modulator is derived fromthe common load resistor 81 by capacity coupling, a condenser 33 is con- 'nected to the junction point of the resistors aforementioned and to the resistor 39 which functions as'the screen grid load resistor 01- the mixer tube 13. The voltage supply source for the screengrid 93 is'obtained from the battery 9!.

input transformer 51 in series with the anode supply battery 33. The secondary winding 53 thereof is center-tapped. the two terminals being connected to the grids 59 and 3| of the tubes 33 land 32, respectively. The battery 33 inseries be tween the interconnected cathodes 33 and 35 and the center tap provides the initial bias source for the grids 59 and 3|. It may be mentioned here.

although further reference thereof will be had,

that the output of the amplifier tube 39 is applied b means of the divided circuit of the secondary In the screen grid circuit of the tube I3 in series with the resistor 39 is also a filter comprising the self-inductance 92 tuned by the condenser 93.

In describing the operation o'fthe system it is important to note that the' frequency of the oscillator 13 is chosen to be higher than the highest audio frequency to be detected, that is, it is a supersonic frequency and may be of any order of magnitude within limits of practical engineering design. Itis also to be noted that this frequency will be the intermediate frequency in the system to which the amplifiers 26 and 23' frequency.

. value.

are sharply tuned Tuned primary andsecon ary circuits, as shown, will have adequate selec tivity to respond only to the intermediate frewith voltages at the difierence frequencies store quency. In certain practical applications, 'patticulariy when the oscillator frequency is chosen, to be-very high in comparison tothe highest audiofrequency to be received, it .may be .ad-

- vantageous to use the crystal-filter type of coupling in the intermediate frequency amplifier.

whereby extreme selectivity to a may be obtained.

For the sake of easier understanding. we may ngle frequency give numerical values to the various frequencies under consideration and by way of example, assume that the irequencyof the drone of the airplane has predominantcomponents around- 500 cycles. It is, of course, immaterial what the predominant audio frequency. emitted by. the

planemay be, since the system will respond to all frequencieswithin the audiorange. The example given here is purely an arbitrary choice in value in order to explain the operation more easily. The 500 cycle note will excite both micromentioned. V r In the mixer tube, we had on the signal grid- !0, the 500 cycle signal whereas on the screen rid 00 we have two signals of frequencies 30,500 and 20,500. Consequently, in the outputv circuit, there will appear signal voltages of the following frequencies: the additive component of the audio voltage on the grid i0 and. the 30,500 cycles resulting in 31,000 cycles, the subtractive component comprising the 500 cycle signal voltage and the 30,500 cycle voltage giving 00,000 cycles. The mixing of .the other component, namely, the 29,500 cycles will also result in the additive relationin 30,000 cycles, whereas in the subtractive relation it will be 20,000 cycles.

Therefore, we have frequencies of theorder of 31,000, 30,000 and 20,000 cycles. these, we have selected in our intermediate frequency amplifier the 30,000 component to which. the amplifier is sharply responsive and this voltage will phones .i and 2 simultaneously when the plane is in the receptive range'of the microphones, as shown in Fig. l. The 500 cycle signal voltage derived from the microphone I will be amplified by the tube 1 and appears at the input, thatis on the grid it of the tube It. The output circuit of this .tube, namely, the -primarywinding fl, is sharply tuned to a much higher frequency outside of the audio frequency range. To continue our numerical example, let us choose 30,000-

cycles for the intermediate frequency. Consequently, the output circuit will be of very low impedance, almost a short circuit for 500- cycles and whatever minute energy may be had at that frequency in the output circuit of,- the tube it, it

will not be amplified further dueto the selective response of the intermediate frequency amplifier to only 30,000 cycles. This illustrates also the fact that any other audio frequencys'igna'l whichv 00 resulting in a unidirectional eminent be amplified and applied to the diode detector,

, magnitude of which depends upon the magnitude of' the signal applied to the system iromthe two -j microphones. The microammeter in this circuit,

if we desire to indicate the current flow, will serve as theindicator of the presence of the audicfres quency vibrations which were simultaneously applied to the two microphones. Of course a volt-- metermay also be used to measure the voltage across the load resistor ti or. other similar indieating instruments, which'will indicate the pres ence of the-intermediate frequency signal in the y may be applied to the microphone lmdueto external noise as long as it does not reach the other microphone. will not produce a' voltage in' the output circuit of the tube ifisince the output I circuit and the succeeding amplifiers do not respend to audio frequencies- As stated before,

the microphone 2 is also energized by the 500 cycle note from th airplane. Similarly in the output of theamplifie'r tube 20.

It is again'emphasized that the above frequency values are entirely arbitrarily chosen and the. samerelationship will hold with. respect to any audio frequency signal to which the microphones are capable of"response. If we indicate the fre q'uency which is simultaneously converted into electrical currents by the microphones as f1 and the intermediate frequency supplied by the oscil later 70 as is, it will be seen that in the output circuit of the balanced modulator, "we will have the frequencies 13+): and fi-fz. In the mixer 7 tube in order that the system shall be responsive,

output circuit ofthe amplifier tube 40, an output voltage at 500 cycles will appear and will be transferred to the grids 09 and 0|. ,Simultane- 'ously the oscillator i0 applies voltage tothe screen grids 08 and diet a frequency elf-30,000

cycles since this was chosen as the intermediate Consequently in 'the output circuitof the balanced modulator, namely, at the June-- tion point of the resistors 83, 85 andtl, the sum and differencefrequencies will appear. These frequencies will be 30,500 cycles and 29,500 cycles. The 30,000 cycle voltage from the oscillator I0 and the 500 cycle signal voltage will cancel out in the output circuit because of push-pull excitation of both sets of controljelectrodes. This holds true when the two tubes and 62 are.

matched and the resistors 03 and 85 of equal To take care of any slight disbalance rid at we must obtain is that is the intermediate fre- 'quency. This is effected by beating the audio signal h with the frequency components obtained from the balanced modulator. This results in the following components:

of these (0) and (c) give the intermediate fraquency is. It is clearly seen that'in accordance with this'invention only such source of audio frequency energy. will be intercepted which simultaneously energizes two microphones widely spaced. Therefore, irrespective of the high gain which-each amplifier following the individual microphone may have. nonoise affecting a single microphone will interferewith the intended operation. It is necessary that the audio signal shall energize both microphones in order to give a beat frequency resulting in the intermediate frequency. I claim as my invention: 1. v In a system for detecting the presence of "aircraft by sound 'vibrationsemanating from the engine on the propeller thereof, sound pickup die vicesspaced at a'substantial distance fromleach other and operable. to translate said sound vibra tions into electrical current variations, a dual channel transmission system for said currents,

means in one only of said channels'for combining ascasae electrical-acoustic type said variations with electrical currentvarlations of a predetermined frequency, means for mixing the resultin combined variations with the said variations from said other channel, and means for selectively amplifying the frequency comp o-- nent corresponding to said predetermined fre- -quency.

2. In a system for detecting the'presence of rents each individually energized by one of said aircraft by sound vibrations emanating from the engine or the propeller thereof, sound pickup devices spaced at ahsubstantial' distance "from each other and operable to translate saidsound vibrations into electrical current variations, a dual channel transmission system for said yaria s-ion" s in one only of 'saidchannels for comhin! r said variations with electrical current vari- V ations generated in said system and having a pre determined frequency, means for mixing the respaced at such distance that. one is not substantially aifected'by sound energy originated in the locality-of the other, said two devices being simultaneously energized by said audio-frequency vibrations, a dual channeltransmission system for audio-frequency curdevices, means for generating currents -.of a fixed supersonic frequency, means for combining the audio-frequency output'energy of one. only of said channels with said fixed frequency whereby to obtain the sum and difference frequencies between said audio-frequency energy and said. fixed supersonic frequency,- means for cancelling said fixed frequency and transmitting said combined frequencies, means for mixing said last-mentioned frequencies with the energy output at said audio frequency of said other channel, theresuiting combined variations with the said variaclone from said other channel, meansfor selectively amplifying the frequency component corresponding to said predetermined frequency, and l. 1" indicating thereiative magnitude of uplifted component. r so, system for detecting the presence of in flight by sound vibrations emanating engine or propeller thereof, a plurality of sound pickup devices, of the electrical-acoustic frequency, and means for rectifying the output 25' by means of audio frequency vibrations emitted by obtaining currents at said fixed supersonic fre quency, means for selectively amplifying "saidf last-mentioned currents comprising a plurality of amplifying "stagessharply tuned to said fixed energy of said amplifier. y Y t 5. In a detecting system for airplanes-in'fiight therefrom, a pair of electroacoustic devices spaced 1 at such distance that one is not affected by sound type spaced at a substantial distance from each and each producing an. electrical current corresponding in frequency to. said sound vibra-' tions, a dual channel electrical transmission system, each channel being individuallyv energized from one of said devices, means in one only of said channels for combining the output energy thereof with a locally generated oscillation of. a fixed frequency, means for transmitting-the resuitant sum and difference frequencies and cancelling said fixed frequency, means for mixing said last-meritioned frequencieswith'the output energy originating in the locality of the other, said devices being so positioned as to be simultaneously energized by said audio frequency vibrations, an amplifier connected to the output of each of said devices, a circuit for connecting one of said amplifiers to the input of a mixer amplienergy having the frequency of saidsounjd vibraitlons of the other of said'channels, thereby obtaining said fixedfrequencmf'and means for se-' lectively amplifyingsaid fixed frequency energy, and means for indicating the amplitude thereof. 4. In a system for detecting aircraft in flight I by audio-frequency vibrations emanating therefrom, a plurality-of sound pickup devices of the I said amplifier. 150

fier, a circuit for connecting the output of said other amplifier to the input of a balanced modulator, an oscillator producing energy at a fixed supersonic frequency, circuit means for combining said last'mentionedenergywith the audio 1 frequency energy in said modulator; circuit means for cancelling said fixed frequency energy from the-output of said modulator and transmitting the resultant sum and difference frequencies to said mixer amplifier, an output circuit for said mixer amplifier, a plurality of cascaded stages tuned to said supersonic frequency energized therefrom and a detector forlndicatin the amplitude of the signal m m A. READ.

currents. produced by 

